A light emitting diode (LED) is a type of semiconductor lighting device increasingly used for virtually any and all type of lighting applications. LEDs include a p-n junction like other solid state diodes. As current flows from the p side of the junction to the n side of the junction and electrons transition into a lower energy state through recombination with a hole. Upon transition, the electrons release light energy. The wavelength of the light emitted at a given junction, and therefore the color of the light emitted, depends upon the bandgap energy of the materials forming the p-n junction. Accordingly, each p-n junction emits light at one wavelength and most LEDs are monochromatic devices.
A LED “green gap” exists covering a wavelength emission range of about 530 nm to 570 nm. Although green emitting LEDs are know, the fabrication of devices having comparable efficiency to LEDs designed to emit in other portions of the spectrum has posed significant technological challenges for the solid state lighting industry. An underlying problem with efficient LEDs emitting in the green gap is the lack of, or the difficulty encountered in creating, semiconductor materials having a suitable bandgaps for emission in the green gap.
Green emitting LEDs have been fabricated, for example, from GaInN and other wide bandgap nitrides. GaInN devices that have a suitable bandgap for emission in the green gap may suffer from reduced efficiency and stability because of poor material quality. The material quality of GaInN devices can be compromised because of alloy phase separation when substantial amounts of In are added to GaN, or when substantial amounts of Ga are added to InN. In addition, the GaN and other substrates required for GaInN devices having a suitable bandgap for emission in the green gap may be relatively expensive.
Alternatively, green emitting LEDs may be fabricated from AlGaInP or other quaternary semiconductor alloys containing aluminum. Presently, aluminum containing devices also tend to be inefficient as a result of short minority carrier lifetimes caused by traps related to oxygen incorporation, especially for devices with high Al content.
LEDs emitting in the green range are desirable for use both as inherently green light sources and as component elements in white light or other mixed color sources. There are two primary ways of producing high intensity white-light with LEDs. The first method is to use a phosphor material to convert monochromatic light from a typically blue or UV LED to broad-spectrum white light, in much the same way a fluorescent light bulb works. This is known as the phosphor conversion (PC) approach. The primary technological challenge presented by a phosphor conversion device is to improve the Color Rendering Index (CRI) of the PC white light output while also maintaining high efficiency. The CRI problem inherent in PC devices may be circumvented altogether by using the second white light generation method, a Red-Green-Blue (RGB) color mixing approach. According to this method, individual LEDs that emit the three primary colors, red, green and blue, provide a mixed output of wavelengths perceived as white light. The color mixing method of white light generation is very flexible. In theory any color output can be produced using red, green and blue sources. The color mixing method is however limited by the challenge of obtaining an LED efficiently emitting in the green gap.
The embodiments disclosed herein are intended to overcome one or more of the limitations described above. The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.